This invention relates to hydraulic circuits for controlling a fluid pressure-operated cylinder, motor or the like and more particularly to systems of this kind which utilize an accumulator to reduce pressure fluctuations which can otherwise interfere with precise control of the fluid motor operation.
Fluid pressure-operated cylinders, actuators or other forms of fluid motor usually have a control system with a control valve through which pressurized fluid from a pump and reservoir may be selectively transmitted to one or more fluid ports of the cylinder and through which fluid may be selectively released from one or more of the cylinder ports in order to actuate and deactuate the device. The fluid cylinder moves a load, such as the load-supporting forks of an industrial lift truck as one example, under conditions where precise positioning of the load by operation of the cylinder may be a highly important consideration. The operator should be able to stop the cylinder smoothly and quickly at a precise point and without bounce or other erratic cylinder motions. Thereafter the cylinder should remain precisely fixed, without creep-up or other forms of drift, until the control valve is again operated.
To aid in realizing this objective it is a known practice to couple an accumulator to the fluid supply passage between the control valve and the cylinder through a unidirectional flow device connected in parallel with a flow restriction. This aids in reducing unwanted pressure fluctuations and surges which can arise upon closing of the control valve to stop cylinder operation. It uncorrected for, such pressure fluctuations can result in a phenomenon known as bounce which causes erratic unwanted cylinder motion and which interferes with precision control of the point at which the cylinder stops upon closing of the control valve.
In these prior systems the excess pressure which could otherwise build up in the accumulator in the process of smoothing out such pressure fluctuations is slowly fed back to the supply passage between the control valve and cylinder through the above-mentioned flow restriction. While the accumulator, the unidirectional flow device and the flow restriction are effective for the intended purpose of alleviating bounce and other undesirable pressure-fluctuation effects, these elements have been found to contribute to a different undesirable effect, known as creep-up, which also interferes with positional stability of the cylinder.
In particular, after the control valve has been closed with the intention of immobilizing the cylinder and load, the excess high pressure from the accumulator slowly feeds back to the cylinder through the flow restriction and causes cylinder movement away from the exact position at which the operator stopped cylinder operation. In other words, drift or creep-up may occur during the period that the control valve is closed. Thus these prior systems alleviate one effect which could interfere with precision control of the cylinder, but in the process another effect is introduced which itself detracts from the desired result.